The aim of this research was to investigate the thermal stability of an ultrafine grained metastable β Ti alloy, Ti–25Nb–3Zr–3Mo–2Sn, which was formed using a modified accumulative roll bonding technique. The thermal stability was assessed during annealing heat treatments through property and microstructure observations. Only small changes were detected in the hardness during annealing at temperatures of 400 °C–500 °C. There was no change in the β grain size after prolonged annealing treatments at 400 °C and limited grain growth at 500 °C. At these temperatures coarsening of the α phase and some recovery is evident. Significant recrystallisation of the β phase in conjunction with grain growth occurred at an annealing temperature of 600 °C and was associated with substantial reduction in the hardness. An ultrafine grain structure was retained after prolonged annealing at this temperature.